Three identical spheres, each having a charge q and radius R, are kept in such a way that each touches the other two. The magnitude of the electric force on any sphere due to the other two is

For external points, a charged sphere behaves as if the whole of its charge is concentrated at its center.Force on A due to B,FAB=14πε0q2(2R)2=14πε0q24R2  along  BA→And force onA due to C,FAB=14πε0q2(2R)2=14πε0q24R2  along  CA→Now as angle between BA and CA is 600 and |FAB|=|FAC|=F∴FA=F2+F2+2FFcos600=3  F=14πε034(qR)2